Welcome to the forum!
Standard questions - have you checked all wiring connections, is everything lubed like it should be, is anything loose, etc.? If you're losing steps it could be one of those or a combination and it could be the drivers, steppers, etc.
Does it do it all the time or only when it has been used a while? Does it do this in the same place each time or is it random? Which axis or are all affected?
David